A computer controlled machine tool has been developed for laser cutting and marking a workpiece wherein the working zone of the machine tool is smaller than the workpiece that can be processed. The workpiece is carried on a translatable worktable or pallet that is indexed through a working zone. There is a small overlap within the work zone at the trailing edge of the first working position and at the leading and trailing edge of subsequent working positions. The leading portion of the workpiece is positioned within the working zone and is processed. The work support is then indexed to position a subsequent portion of the workpiece in the work zone for processing. This configuration creates interrupted cuts wherein a cut is stopped in one processing zone and must be reestablished in a subsequent processing zone. Registration of the workhead guidance system relative to the workpiece must be maintained along the entire length of the workpiece to accurately produce a part.
The work support carrying and guidance system is such that the position of the work support is repeatable but its motion is not perfectly straight. Locating pins locate the work support, also called a worktable or a pallet, final position but the locating pins cannot assure that registration of the workhead guidance system relative to the workpiece is maintained. The workpiece rests freely on the work support. The workpiece is not clamped in any way. The workpiece can shift slightly on the work support during processing, due to thermal effects, and the workpiece can shift on the work support when the work support is indexed.
Known methods of locating a part on a laser machine tool are not entirely satisfactory in addressing this registration problem. For example Klingel et al. U.S. Pat. No. 5,132,510 discloses a workpiece position sensing assembly 120 (col 6, lines 6–7) including a depending sensor 122 that extends below the upper surface of the workpiece 38 (col 6, lines 14–17), to measure or sense the position of the workpiece (col 6, lines 17–22), by measuring the position of at least two cutouts (col 8, lines 8–27). The apparatus is further described in Kilian et al. U.S. Pat. No. 5,304,773 at col 1, lines 42–52 as a sensor probe and as a mechanical probe. Measuring probes are available in a variety of forms from RENISHAW.
Kilian et al. U.S. Pat. No. 5,304,773 discloses an optical sensor assembly (col 2 lines 19–27), which measures the position of a workpiece by sensing passage of a light beam over the edge of a reference formation (the edge of a hole or shape cut into the workpiece) (col 2, lines 34–42). Col 8, lines 48–53 describes the first step in using the sensor assembly is to cut a square hole for calibration of the guidance system for a workpiece of a new thickness and carefully remove the internal cutout (col 8, lines 59–60).
Laser workhead nozzle capacitive sensor height controls are available, but are found lacking as a measuring probe or optical sensor.
The objection with all above described measuring methods is all require a hole in the workpiece. The hole must be of sufficient size for the measuring method utilized. For best accuracy two widely separated holes should be measured. There is no assurance the workpiece will have holes within the overlap work zone. Requiring holes be cut within the overlap work zone may increase the amount of scrap. It cannot be assured that the internal cutouts will fall from the holes. The internal cutout must fall free or must be removed before the hole can be measured. Should a workpiece shift while a cutout is being removed registration of the workpiece will be lost.